1. Field of the Invention
This invention relates to a process for producing an A-15 type superconductor compound, and more specifically to a process for producing an A-15 type superconductor compound having the composition A.sub.3 B composed of niobium (element A) and at least one element (B) selected from the group consisting of gallium, aluminum and germanium.
2. Description of the Prior Art
A superconductor material having an electric resistance of zero can permit flowing of a large current without power consumption, and the superconducting state is maintained even in a high magnetic field. Because of this characteristic, superconductor materials have increasingly come into use as coil materials for electromagnets generating a high magnetic field which are used, for example, in an NMR analysis device, an energy storage device, a nuclear reactor, and a particle accelerator for high-energy physics.
An Nb-Ti alloy and Nb.sub.3 Sn and V.sub.3 Ga compounds called A-15 type compounds are now used as a superconductor material for coil materials used in electromagnets capable of generating a high magnetic field.
The Nb-Ti alloy has high plasticity and can be directly worked into a wire or tape. Nb.sub.3 Sn and V.sub.3 Ga, the A-15 type compounds, are hard and brittle and cannot be directly worked. Hence, working of these A-15 type compounds into a wire or tape relies on a diffusion reaction by a surface diffusion method or a so-called bronze method. For example, Japanese Patent Publications Nos. 21356/1972 and 3038/1974 disclose the production of a superconductor by the surface diffusion method.
The upper critical magnetic fields H.sub.c2 of Nb-Ti, Nb.sub.3 Sn and V.sub.3 Ga at 4.2 K. are 12 T, 21 T and 22 T (tesla), respectively. As the applied magnetic field approaches the upper critical magnetic field, the critical current density rapidly decreases. Accordingly, even when a V.sub.3 Ga wire is used, the magnetic field generated by a superconducting magnet is 17.5 T at the highest. On the other hand, as the utility of superconductors has been developed, the superconducting magnets have been required to generate a stronger magnetic field, and it has been desired to develop superdonducting wires or tapes having higher performance. For example, it it said that a superconducting magnet for Mirrer-type fusion reactors is required to generate a magnetic field of 20 to 24 T. Such a high magnetic field is difficult to obtain by existing Nb.sub.3 Sn and V.sub.3 Ga. Nb.sub.3 Ga (H.sub.c2 =34T), Nb.sub.3 Al (H.sub.c2 =30T) and Nb.sub.3 (Al, Ge) (H.sub.c2 =41T), which all fall into the category of A-15 type compounds, are possible candidates for a material capable of generating such a high magnetic field. If, however, such materials are to be produced by the conventional surface diffusion method, the heat-treating temperature must be very high. Consequently, the crystal grains become large, and the critical current density Jc of the resulting materials, which is of importance in practical applications, is markedly reduced. When the bronze method using a copper-base alloy is applied to these materials, the high heat-treatment temperatures used result in dissolving of copper in the superconductor phase. As a result, the resulting conductors have deteriorated superconducting properties, and become unsuitable for practical applications.